Metals with inhomogeneous magnetic field strength

ABSTRACT

Metal nanowires were formed on electroformed magnetic metals when porous templates were prepared by anodizing aluminum or its alloys. The aspect ratio of the anodic oxide pores was controlled by the electrolyte composition, electrolytic conditions, and the subsequent widen-treatment of the anodized pores. Because of the magnetic field strength of the electroformed nanowires increases with increasing the aspect ratio, the electroformed metal with higher aspect-ratio nanowires has a greater magnetic field strength. Accordingly, metals with inhomogeneous magnetic field strength were obtained.

BACKGROUND OF THE INVENTION

The present invention relates to metals with inhomogeneous magnetic field strength made by electroforming process, especially to magnetic metal nanowires formed on one side or part of electroformed magnetic metals by using anodic porous alumina or its alloys with hexagonal cellular structure as templates. The electroformed metal with higher aspect-ratio nanowires has a greater magnetic field strength so that metals with inhomogeneous magnetic field strength were obtained.

Applications of magnetic material to electronic and biochemical industries are getting more important. For example, they can be applied to high density magnetic data storage device or thin-film magnetic head with high coercivity.

Moreover, the invention of nano science and technology has created new generation of technology and applications. In new era of nano technology, well-know nano-scale anodic porous alumina induces new applications and development. The pores on oxide film of aluminum or its alloys according to the present invention are mainly in hexagonal shape. However, the pores formed may be not ideal and deformed into pentagonal or quadrangular. But in industries or scientific essays, they are still seen as hexagonal. Thus porous oxide film of aluminum or its alloys is called “ordered hexagonal anodic porous alumina” in the present invention. In conventional industries, ordered hexagonal porous oxide film formed in suspended in acidic electrolyte with aluminum anode has feature of high hardness, abrasion resistance and a corrosion resistance. Thus aluminum anodizing has been commonly used in industry for several decades. Besides electrolytic conditions, the pores size has been influenced by electrolyte composition. By using different acidic electrolyte in combinations with different conditions of electrolysis, pores with various diameters ranging from 5 to 250 nm are obtained. And the length of the pore can be as long as hundreds of nanometers. Thus nano-scale pores with different aspect ratio are obtained and the aspect ratio can be changed by subsequent pore-widening treatment. Apart from aluminum, its alloys can also get similar anodizing results and be applied according to the present invention. Refer to Journal of Applied Physics, page 4721, Vol. 87(9), 2000, magnetism works as aluminum so that Al—Mg alloy anodizing is similar to that of aluminum. In addition to Al—Mg alloy, in Electroplating published by McGraw-Hill, NY, F. A. Lowenheim points that aluminum alloys for anodizing include Al—Mg—Si alloy, Al—Zn—Mg alloys, Al—Mn alloy, Al—Mn—Cr alloy and Al—Mg—Si—Cu—Cr alloy.

The barrier layer of the above nano-scale porous oxide film is thin and conductive so that the porous oxide film can be used as the template to prepare the nanoparticles or nanowires. For example, in Electrochimica Acta, page 145, volume 42, 1997, silver nano-particles are formed from pores of ordered hexagonal porous oxide film. This manufacturing process can also be applied to electroplating of magnetic material. For example, refer to p. 241, Vol. 249, 2002, Journal of Magnetism and Magnetic Materials, iron-cobalt and nickel magnetic nanowires are electroplated from the pores on oxide film by means of alternating current. The hysteresis loop of these nanowires is also discussed therein. Magnetic metal is electroplated inside the ordered hexagonal porous oxide film and the porous film of alumina or its alloys are removed so as to get nanowires electrodeposited inside the pores. These magnetic nanowires with high aspect-ratio show specific magnetic field strength. Refer to p. 1340, Vol. 37, 1998, Japan Journal of Applied Physics, it points that the magnetic filed strength of these magnetic nanowires increases along with their aspect-ratio. By means of ordered hexagonal porous oxide film with various aspect-ratios, nanowires with various magnetic filed strength can be obtained.

The present invention uses nano-scale pores of the ordered hexagonal porous oxide film of alumina or its alloys with various aspect-ratios as template for eletroforming so as to produce electroformed metals with one side or selected area having magnetic nanowires. Thus one side or selected area of the electroformed metals has higher magnetic field strength. This magnetic field strength increases along with the increasing aspect-ratio. Therefore, electroformed metals with inhomogeneous magnetic field strength are obtained according to the present invention for applications to electrical and biochemical industries.

SUMMARY OF THE INVENTION

It's complicated to generate inhomogeneous magnetic field strength on selected area of surface on metal. The feasible ways are by electroplating or other physical coating methods to coat magnetic materials with different magnetic strength on surface of metal. However, such method has a blind spot, especially when the metal has special shape such as tubular metal, it's impossible to get higher magnetic field strength on specific area inside the tubular metal. By means of electroforming used in industries for several decades, metals with inhomogeneous magnetic field strength are produced for applications to electronic and biochemical industries and further inducing development of new inventions.

As mentioned above, the electroplating of nano-scale hexagonal cellular anodic porous alumina or its alloys has been developed by the nano science and technology. The diameter of pore of oxide film ranges from 5 to 250 nanometers while the size of the diameter increases along with the increasing of the applied voltage, in direct proportion. 2.5 nanometer/V. For example, higher voltage is applied in phosphoric acid electrolyte for obtaining pores with larger diameter and the depth of the pore extends along with reaction time of electrolysis. The pore can also be widening by change of other conditions. By adjusting aspect ratio of the pores on oxide film, magnetic nanowires with various aspect ratios are obtained by electroplating. Thus magnetic nanowires with different magnetic field strength are obtained. Under fixed temperature, the retentivity and the coercivity are obtained by detection of hysteresis loop of the magnetic nanowires. The present invention uses ordered hexagonal anodic porous alumina or its alloys as a template for electroforming so as to obtain metals with nanowires. That means the metals have nanowire structure on selected surface area. Due to different magnetic filed strength caused by various aspect ratios of nanowires, metals with inhomogeneous magnetic field strength were obtained.

Aluminum or its alloys are conventional material for electroforming template, easy to be processed into various shapes, and to be removed by alkaline solutions. In combination with manufacturing of magnetic nanowires, various metals are electroformed and so as to have inhomogeneous magnetic field strength.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein

FIG. 1 is SEM micrograph at 10000× magnification of the surface view of anodic porous alumina; the second anodizing process was conducted in 0.6 M phosphoric acid for 2 hours;

FIG. 2 is SEM micrograph at 10000× magnification of the surface view of anodic porous alumina (10000×); the second anodizing process was conducted in 1 M phosphoric acid for 7.5 hours;

FIG. 3 is SEM micrograph of the surface view of hexagonal cellular nickel nanowires with aspect-ratio of 155 (10000×);

FIG. 4 is SEM micrograph at 30000× magnification of the surface view of hexagonal cellular nickel nanowires with aspect-ratio of 155;

FIG. 5 is a schematic drawing of the embodiment in FIG. 4, showing hexagonal cellular nickel nanowires 11 on the electroforming nickel 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention mainly includes manufacturing of templates and electroforming of magnetic metals. The manufacturing of template for fabricating porous anodic alumina not only needs to control the aspect-ratio of the porous anodic alumina but also to adjust conductivity on surface of the template. The electrolyte for anodizing includes sulfuric acid, oxalic acid, and phosphoric acid, each applied with different power supplies ranging from 10 to 200 volts. For example, the phosphoric acid requires higher voltage so as to get ordered hexagonal pores on films. A two-stage anodizing process is used by the present invention. Remove the obtained oxide films aluminum or its alloys by the first stage and get more integrated hexagonal anodic porous film by the second anodizing process.

General temperature for electrolyte solutions of anodizing ranges from zero to 35 Celsius degrees. During the anodizing process, the solution needs to be thoroughly stirred for efficient heat dissipation. While widening pores by acid solutions, the diameter of pores can be enlarged by adjusting concentration of the acid solutions, processing time and the temperature. The pore size and thickness of alumina film can be measured by scanning electron microscopy (SEM). The shape and area for anodizing of the aluminum or its alloys can be adjusted in advance according to users needs and the conductivity of the template can be improved by sputtering or chemical vapor deposition. Aluminum, lead, stainless steel, and other insoluble electrodes make most convenient cathodes for anodizing process. Moreover, besides general conditions, wetting ability of electrolytic solution needs to be controlled. The electrolytic solution with higher wetting ability can penetrate into nanometer-scale porous film so as to produce electroformed metals with nanometers wires. After electroforming process, alkaline solution is used to remove aluminum or its alloys and their oxide films. Besides morphology and size of magnetic nanowires observed by scanning electron microscope, the magnetism is measured by gauss meter. The composition of nanowires is investigated by energy dispersive X-ray spectrometer (EDS) while X-ray powder diffraction (XRD) is used to identify crystal form of metal nanowires. The microstructure of the metal nanowires shows higher crystallinity.

Embodiment 1

After being treated with electrolytic polishing and mechanical polishing, aluminum sheet with size of 20×20×7 mm and purity of 99.9% is taking the first anodizing step at 161 volts for an hour with 1.0 M phosphoric acid at 0 Celsius degrees. The processed oxide layer is put into 0.5 M phosphoric acid at 45 Celsius degrees for 30 minutes for oxide-corrosion. Then take the second anodizing step at 161 volts for 2 hours with 0.3 M or 0.6 M phosphoric acid at 0 Celsius degrees. Insoluble iridium oxide is used as a cathode in both of the first and second anodizing steps. The obtained oxide film is put into 0.5 M phosphoric acid at 45 Celsius degrees for 15 minutes to increase the pore size and anodic porous alumina with ordered hexagonal pore arrays is obtained. FIG. 1 is a SEM micrograph of surface of anodic porous alumina at 1000× magnification, the diameters of these pores are about 250 μm. The ordered hexagonal cellular pores in anodic porous alumina are suitable for subsequent electroforming.

Embodiment 2

Anodization condition is the same as embodiment 1, concentration of the phosphoric acid in a second anodizing increases to 1 M and the treatment time also increases to 7.5 hrs. Refer to FIG. 2, a SEM micrograph of surface of anodic porous alumina at 10000× magnification, the diameter of the pore is about 400 nanometer while the height (thickness) is about 62 μm. Thus the aspect ratio (height to diameter) is 155, which is suitable for subsequent electroforming.

Embodiment 3

Anodization condition is the same as embodiment 1, but the acidic electrolyte is replaced by oxalic acid. Under condition that the concentration of oxalic acid ranges from 0.3 M to 6 M, applied voltage from 20 to 40 volts, and time for the second anodizing from 1 to 4 hrs, various anodic porous alumina with smaller pores are obtained. For example, diameter of the pore is about 90 nanometer while second anodizing in 0.3 M oxalic acid at 40 volts. This porous alumina is also suitable for subsequent electroforming.

Embodiment 4

Anodization condition is the same as embodiment 1, but the acidic electrolyte is replaced by sulfuric acid. In a series of anodizing processes, the concentration of sulfuric acid ranges from 0.3 M to 1M, applied voltage from 10 to 30 volts, and time for the second anodizing from 1 to 5 hrs, anodic porous alumina as embodiment 1 with much smaller pores is obtained. For example, diameter of the pore is about 45 nanometer while second anodizing in 0.3 M sulfuric acid at 25 volts. This porous alumina is also suitable for subsequent electroforming.

First Sample for Comparison:

Anodization condition is the same as embodiment 1, but the acidic electrolyte is replaced by boric acid or tartaric acid. After a series of anodizing processes with different electrolyte compositions, electrolyte concentration, voltage power supply, and length of time, the anodic porous alumina with ordered hexagonal cells still can't be obtained. Thus there is no anodic porous alumina that can be served as a template for subsequent electroforming.

Embodiment 5

Anodization condition is the same as embodiment 1 and the anode is made from Al—Mg alloy. The anodic porous alumina is obtained and is suitable for subsequent electroforming

Embodiment 6

Anodization condition is the same as embodiment 2, but the treatment time for second anodizing in 1M phosphoric acid is only 1 hr so that the pores have lower depth, only about 11.6 μm. And the aspect-ratio is only 29. The barrier layer of anodic porous alumina is not thick and can be formed by electrodeposition. But the surface layer of the anodic porous alumina is not a good conductor, resulting in the anodic porous alumina more close to the surface layer is less conductive. Thus the surface of the anodic porous alumina is sputtering with gold for 30 seconds so as to overcome the nonconductivity of the surface layer and so as to make whole the film work as template for electroforming. For nickel electroforming, this template is set in electrolytic solution with nickel sulfamate and boric acid and concentration of wetting agent is 0.2 g/l of 50 Celsius degrees at current density of 1 A/dm² for 6 hours. Al and alumina film are removed in 10% sodium hydroxide solution and thus the electroformed nickel is obtained with nickel nanowires on one side. The composition of the nickel nanowires is identified by energy dispersive X-ray spectrometer (EDS). In X-ray diffraction (XRD), the peak of the (111), (200) and (220) crystal planes show high crystallinity. The nanowires with high crystallinity has more uniform magnetic field showing higher intensity of a magnetic field. Therefore, the strength measured on the magnetic filed of electroformed nickel with nickel nanowires is 2.4 gauss.

Second Sample for Comparison:

Nickel electroforming is the same as the embodiment 6, but the aluminum template has not been anodized and that means the aspect-ratio is zero. Thus the electroformed nickel obtained as that of embodiment 6 includes no nickel nanowires and only magnetic field strength of the nickel itself shows. The measured strength is only 0.6 gauss.

Embodiment 7

Use the anodic porous alumina as in embodiment 2 and the same nickel electroforming process as embodiment 6 is practiced, but the treatment time for second anodizing is extended to 7.5 hrs so that the pore depth is 62 μm. And the aspect-ratio is increased to 155. On one side of the electroformed nickel has nickel nanowires with higher aspect-ratio. FIG. 3 is a SEM micrograph of the surface view of hexagonal cellular nickel nanowires with aspect-ratio of 155 (1000×) obtained after Al and anodic porous alumina being removed by 10% sodium hydroxide solution, showing ordered hexagonal cellular nickel nanowires. FIG. 4 is SEM micrograph at 30000× magnification, showing the ordered hexagonal cellular nickel nanowires more clearly. FIG. 5 is a schematic drawing of the FIG. 4. Because magnetic nanowires with higher aspect-ratio has higher magnetic field strength, the magnetic field strength measured on electroformed nickel with nickel nanowires is increased to 21.6 gauss. Compared with the embodiment 6, it is obvious that the magnetic field strength of nanowires on magnetic metal relates to its aspect-ratio and is in direct proportion. Therefore, the magnetic field strength of electroformed metal parts changes by changing the aspect-ratio of the anodic porous alumina so as to obtain metals with inhomogeneous magnetic field strength.

Embodiment 8

Use the anodic porous alumina as in embodiment 2 and the same nickel electroforming process as embodiment 7 is practiced, but part of the surface of the aluminum is coated with tapes for stop off before aluminum anodizing. Thus only part of the surface of the aluminum is anodized. Also only part of the obtained electroformed nickel includes nickel nanowires. Therefore, only part of surface of the electroformed nickel parts has higher magnetic field strength and that's metal with inhomogeneous magnetic field strength.

Embodiment 9

Use the anodic porous alumina as in embodiment 2 and the same nickel electroforming process as embodiment 7 is practiced, but the aluminum sheet for anodizing is a cylinder with diameter of 3 mm and length of 6 cm. Before aluminium anodizing, tapes with width of 2 cm is attached on top and bottom ends of the aluminum cylinder so that there are only 2 cm central circular area for anodizing process. After anodizing, remove the tape and practice electroforming. After nickel electroforming, tubular electroformed nickel is obtained after removing aluminum and anodic porous alumina. This tubular part is a metal with inhomogeneous magnetic field strength in central 2 cm circular area (anodized area).

Embodiment 10

Anodic porous alumina is got by anodizing in sulfuric acid as the embodiment 4 at 10 V. After pore-widening treatment in phosphoric acid, the diameter of the pore is 20 nanometers with length of 600 nanometers. Moreover, cobalt electroforming is conducted in solution of cobalt sulfate and boric acid so as to get cobalt nanowires with aspect-ratio of 30 on surface of cobalt. The coercivity measured is as high as 1550 oersteds.

Embodiment 11

Anodic porous alumina is got by anodizing in oxalic acid as the embodiment 3 at 20 V. After pore-widening treatment in phosphoric acid, the diameter of the pore is 50 nanometers with length of 350 nanometers. Moreover, iron electroforming is conducted in electrolytic solution of ferrous sulfate and boric acid so as to get iron nanowires with aspect-ratio of 7 on surface of iron. The coercivity measured is as high as 1250 oersteds.

Third Sample for Comparison:

Iron electroforming as the embodiment 11, but the aluminum template has not been anodized and that means the aspect-ratio is zero. Thus the electroformed iron obtained as that of embodiment 11 includes no iron nanowires and only magnetic field strength of the iron itself shows. The measured strength is only 100 oersteds.

Embodiment 12

Use the anodic porous alumina as in embodiment 2 and the same electroforming process as embodiment 6 in electrolytic solution with nickel sulfamate, ferrous sulfamate and boric acid to get Ni—Fe permalloy with the magnetic filed strength of 30.6 gauss.

Embodiment 13

Use the anodic porous alumina as in embodiment 2 and the same nickel electroforming process as embodiment 7 in electrolytic solution with nickel sulfamate, boric acid and wetting agent for 2 hrs and then cleaned for gold sputtering to enhance the conductivity of the surface layer of the anodic porous alumina. Later perform copper electroforming in acidic copper sulfate solutions to get electroformed copper with thickness of 60 μm and nickel nanowires on one side after Al and anodic porous alumina being removed by 10% sodium hydroxide solution. That means the copper part has high magnetic field strength on selected area. The composition of the nickel nanowires is identified by energy dispersive X-ray spectrometer (EDS). Furthermore, the side of the copper part with nickel nanowires can be copper plated to increase the copper thickness to embed the nickel nanowires and being polished so as to make the end of the nickel nanowires as flat as the surface of the copper part. Thus the nickel nanowires with high magnetic field strength is embedded inside the copper part so as to form metal parts with inhomogeneous magnetic field strength.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

1. A method for manufacturing electroformed metals with inhomogeneous magnetic field strength, wherein the electroformed metals are made by using ordered hexagonal porous anodic oxide of aluminum or its alloys generated from anodization in acidic electrolyte as templates.
 2. The method as claimed in claim 1, wherein the electroformed metals are magnetic iron, cobalt, nickel, gadolinium, dysprosium, samarium or their alloys.
 3. The method as claimed in claim 1, wherein the ordered hexagonal porous anodic oxide is able to be electroformed with other metals subsequently after part or whole of pores thereof being electroformed with magnetic iron, cobalt, nickel, gadolinium, dysprosium, samarium or their alloys.
 4. The method as claimed in claim 1, wherein the ordered hexagonal porous anodic oxide is on whole of or selected area of surface of the aluminum or its alloys.
 5. The method as claimed in claim 1, wherein the acidic electrolyte is sulfuric acid, oxalic acid, phosphoric acid or mixtures of above acids.
 6. The method as claimed in claim 1, wherein current for anodizing is direct current or alternating current.
 7. The method as claimed in claim 1, wherein the aluminum alloy for anodizing includes Al—Mg alloy, Al—Mg—Si alloy, Al—Zn—Mg alloys, Al—Mn alloy, Al—Mn—Cr alloy and Al—Mg—Si—Cu—Cr alloy.
 8. The method as claimed in claim 1, wherein aspect-ratio of the hexagonal porous anodic oxide obtained from anodization is controlled by composition of acidic electrolyte, voltage and current for anodizing, temperature of anodizing solution, processing time of anodizing and conditions of pore-widening. 